The ferroelectric materials in FeRAM (Ferroelectric Random Access Memory) and high K materials in DRAM are generally annealed at high temperatures (500 C or above) in oxygen ambient to recover from process damage, e.g. damage caused by the Reactive Ion Etching (RIE) of the materials. During the anneal, the ambient oxygen diffuses into the ferroelectric material and reacts at the high temperature to bring about the recovery. Usually a barrier layer (e.g. Al2O3) is deposited prior to the annealing in order to prevent the ferroelectric from decomposing at the elevated temperatures. When PZT is used as the ferroelectric, it is important that the barrier reduce the outdiffusion of the lead from the PZT.
The barrier layers of the prior art also need to prevent hydrogen from diffusing into and damaging the ferroelectric during, for example, back-end BEOL. Some examples of BEOL processes include insulator deposition, copper processing and forming gas anneal. The barrier layer can be optimized to prevent the diffusion of hydrogen into the ferroelectric, but this optimized layer will also result in a barrier against the oxygen. With less oxygen diffusing into the ferroelectric, the result is a less effective recovery anneal.
One option is to continue the recovery anneal for a longer time at a higher temperature, but this causes problems, especially for capacitors having capacitor on plug structures. Poly silicon plugs or tungsten plugs (contact plugs) are often used as vertical interconnects to connect the bottom electrode of the ferroelectric capacitor to the transistors. In ferroelectric capacitors such contact plugs form a capacitor on plug (COP) structure. In COP structures, a barrier at the top of the plug prevents oxygen from passing from the ferroelectric to the plug. If the recovery anneal proceeds for too long at a high temperature then the oxygen will pass through that barrier at the top of the plug and will oxidize the plug material causing the contact to fail.
As for the PZT, it would be desirable to have a barrier that would allow oxygen to pass into the ferroelectric during an anneal pass while also preventing the ferroelectric from decomposing due to the high temperatures of the anneal. It would additionally be desirable for the barrier to prevent hydrogen from entering and causing damage to the ferroelectric during BEOL processing.